When my son was in 3rd grade, I bought the book: "Robot Building for Beginners", by David Cook. It was still too advanced for him to tackle alone, but we made a point of getting together for a couple hours each weekend and worked through the line-following robot project that the book describes.
It took about two months to complete at that rate, but it was a blast and we both got alot out of it. He got to learn how to breadboard a circuit, cut/drill metal, control an electric motor, fry LEDs;-), and assemble the final bot. And the circuitry itself was so simple that at the end he *really* understood how it worked.
I really recommend this approach, if you've got the time and patience to work with your kid on it.